Most of us think we know exactly what we mean when we use the word “mammal”—and most of us are wrong. Typically, we think only of the sub-group of mammals like us, the so-called placental mammals. There are two other kinds, however: the egg-laying monotremes, which include the duck-billed platypus; and the marsupials, which count kangaroos, opossums and wombats among their ranks. But unless you live in Australia and a few other spots, the vast majority of mammals you run into, even at the zoo, are placentals, a group that encompasses everything from rats to rhinos, gerbils to giraffes, chipmunks to chimps, and, of course humans as well.

It wasn’t always thus, however. Mammals have been around for hundreds of millions of years, but placentals for only tens of millions. Now a new paper just published in Science purports to pinpoint their, or rather, our, origins with impressive specificity. The great-great grandfather of us all, argue the authors, was a small, scurrying, insect-nibbling creature that arose a mere 200,000 to 400,000 years after the cataclysmic extinction event 65 million years ago that wiped out the dinosaurs (or, more precisely, the non-avian dinosaurs, since birds are now considered the one branch of the dinosaur family that survived).

This may seem like just a number to you and me but for paleontologists and evolutionary biologists, it’s something of a bombshell. The prevailing wisdom since the 1990’s, based on assumptions about how quickly mutations arise in DNA, was that the placentals emerged and began to diversify a whopping 35 million earlier, spurred by the breakup of the giant continent Pangea into the smaller landmasses that exist today. They didn’t really flourish until the dinosaurs went away — but then, who could, with huge, voracious lizards towering overhead?

There matters stood until five years ago, when a team of scientists began working on trying to understand the evolution of mammals with funding from the National Science Foundation’s “Assembling the Tree of Life” program. Like their predecessors, says co-author Michael Novacek, provost for science and a curator of paleontology at the American Museum of Natural History in New York, “we did use genomics,” or the mammalian gene-line as a sort of bread-crumb-trail that would lead back to our ancestors.

But they also factored in phenomics – analysis of the body structures, biology and even behavior of creatures both living and extinct, looking for points of similarity that suggest how closely two species might be related. “If we have a fossil that falls in between two living species,” says lead author Maureen O’Leary, a vertebrate paleontologist at Stony Brook University, on Long Island, NY, “we use the living species to form the simplest hypothesis for what the fossil’s behavior was like. Things like ‘does it nurse its young, and if so, for how long? Does does it swim, does it eat insects, is it active at night or during the day?’”

All of that information went into a gigantic Web-based database called MorphoBank, in which the scientists recorded more than 4,500 individual traits, both physical and behavioral, culled from 86 placental mammal species, about half of them extinct. “I was working in one cell [of the database],” recalls Novacek, “when another was suddenly populated with information that was being entered in real time by a collaborator from Brazil. It was really exciting.”

MorphoBank established the species relationships among placental mammals, while DNA and fossil-dating calculations established the timeline — and while that sounds simple enough, it was, says Novacek, “hugely laborious.” Indeed, writes Duke University biologist Anne Yoder in a Science commentary, “O’Leary et al.’s study offers a level of sophistication and meticulous analysis of morphological and paleontological data that is unprecedented.”

All that impressive brainwork led us back to a rather humbling place: You, your loved ones and your friends, not to mention Abe Lincoln, Winston Churchill, Napoleon, Babe Ruth, Marilyn Monroe—all of us, in other words—are the multi-multi-generational grandkids of a rat-like, half-pound, furry-tailed bug-eater. Like it or not. The work, Novacek promises, will go on. “This thing will continue to grow like an organism. We have this important new result, but we also have a playground for future research.” The science may advance, but our egos may never recover.